Numerous medical devices have been developed for implantation or insertion into patients. Unfortunately, many such medical devices are commonly associated with patient discomfort or pain after being positioned within the patient. As a specific example, ethylene vinyl acetate (EVA) copolymer based ureteral stents are widely used to facilitate drainage in the upper urinary tract (e.g., drainage from the kidney to the bladder), for example, following ureteroscopy, endourerotomies, and endopyelotomy for ureteral strictures, as well as in other instances where ureteral obstruction may occur.
A schematic diagram of an exemplary stent 10 of this type is illustrated in FIG. 1. The stent 10 has a proximal end 10p and a distal end 10d. It is a tubular polymer extrusion having a shaft 12, a distal renal retention structure (e.g., renal “pigtail” 14), and a proximal retention structure (e.g., bladder “pigtail” 16). These retention structures prevent upward migration of the stent toward the kidney or downward migration of the stent toward the bladder. Other examples of retention structures for use in ureteral stents include, for example, spirals, coils, corkscrews, mallincotts, barbs, mushrooms and hook ends, among others. Once properly deployed in the ureter, the stent 10 provides ureteral rigidity and allows the passage of urine.
The stent 10, as exemplified by FIG. 1, may further be provided with any one or more of the following: (a) a tapered tip 11, to aid insertion, (b) multiple side ports 18 (one numbered), which are typically arranged in a spiral pattern down the length of the body to promote drainage, (c) graduation marks 25 (one illustrated), which are normally used for visualization by the physician to know when the appropriate length of stent has been inserted into the ureter, and (d) a suture 22, which aids in positioning and withdrawal of the stent, as is known in that art.
During placement, such ureteral stents 10 are typically placed over a urology guide wire, through a cystoscope and advanced into position with a positioner. Once the proximal end of the stent is advanced into the kidney/renal calyx, the guide wire is removed, allowing the pigtails 14, 16 to form in the kidney 19 and bladder 20, as shown in FIG. 2. The renal pigtail 14 of the stent may be closed or tapered, depending on the method of insertion (e.g., the use of a guide wire or otherwise). As shown in FIG. 2, the stent 10 extends through the ureteral orifice 21a and into the bladder 20. For clarity, the ureter entering bladder 20 through the opposite ureteral orifice 21b is not shown.
Such stents are known, however, to be associated with a degree of pain and/or discomfort, particularly in the bladder and flank area after insertion. One way of addressing this pain is to use a softer material, particularly in forming the proximal end of the stent, which engages more sensitive tissue. Stents of this type may employ co-extrusion to combine a firm durometer EVA copolymer at the distal end, which improves stent advancement, and a soft durometer EVA at the proximal end, which improves comfort. A specific example of such a stent is the Polaris™ Dual Durometer Percuflex® Ureteral Stent with HydroPlus™ Coating, available from Boston Scientific, Natick, Mass., USA.
Other ways of addressing pain and discomfort include providing systemically administered painkillers or providing devices which release painkillers locally. See, e.g., U.S. Pat. App. Pub. No. 2003/0224033 entitled “Implantable or insertable medical devices for controlled drug delivery.”
Another issue associated with ureteral stents is the formation of encrustation in vivo, which may be addressed, for example, through the use of devices that release antimicrobial compounds locally. In this regard, see, e.g., U.S. Pat. App. Pub. No. 2004/0249441 entitled “Implantable or insertable medical device resistant to microbial growth and biofilm formation.”